The invention in this case relates to a new and improved method of etching indium tin oxide films.
Finely patterned transparent, conductive layers are used in a number of optoelectronic and photovoltaic devices. These devices include displays, particularly actively addressable displays, heterojunction solar cells, color image pick up tubes, and electrochromic display devices.
Particularly for use in actively addressable displays and in heterojunction solar cells it is required that precisely defined patterns be formed, of line width and spacings of 2-4 microns or less exhibit a low sheet resistance of the order (.ltoreq.20 ohms/square), an ohmic contact to the underlying substrate and good step coverage.
Recently, attempts have been made to form such patterns from thin indium tin oxide films. Thin indium tin oxide films may be readily produced by a number of techniques including reactive sputtering of a In:Sn target in O.sub.2, direct sputtering of a In.sub.2 SnO.sub.3 SnO.sub.2 target, reactive evaporation, ion beam sputtering and thermal evaporation. However the most useful has been shown to be direct sputtering from a target. By this method satisfactory thin films of indium tin oxide are readily produced.
However, attempts to produce fine line patterns (of the order of 2-4 microns or less) by known etching methods have not proved to be successful.
Thus, while wet etching methods including the use of such etchants as HI, HCL, oxalic acid, HCL with zinc powder dispersions and mixtures of HCl and HNO.sub.3 can be used to etch indium tin oxide films, it was found that only mixtures of HCL and HNO.sub.3 can be used to produce fine line etching of indium tin oxide films, a typical etch composition being 2:10:.04 by volume of HCl:H.sub.2 O:HNO.sub.3.
It frequently has been found, however, that use of such an etch presents the problem in that frequent overetching results in breakage of the lines of the pattern.
Attempts have been made to employ plasma etching methods. The use of plasmas containing a gaseous carboxylic acid as shown in Japanese Patent No. 5902004 and plasmas containing a gaseous fluorine compound such as fluorine, hydrogen fluoride and carbon tetrafluoride shown in Japanese Patent No. 5903003 have not proved to be successful in that incomplete etching results due to the production of non-volatile compounds such as indium triflouride which stops further etching of the indium tin oxide and resultant production of electrical shorts have been found to result from the use of such etching method. In addition, the presence of an organic material, the carboxylic acid, in the plasma or carbon tetrafluoride results in the contamination of the device being produced.
Also, attempts have been made to etch indium tin oxide films by reactive ion etching in a plasma formed from argon containing an alcohol such as ethanol or methanol as shown in Japanese Disclosure No. 1985- 234, 325. However, such methods have proved to be unsuccessful probably because organic side products are produced which contaminate the surface being etched and prevent proper etching.